In a case where measurement is taken on the light reflectivity or light transmittance of a plurality of samples being carried, measurement errors may result from the variations of ambient temperature, intensity of light source, sensitivity of a detector, curvature of an optical fiber and the like, which will occur during a sample measurement. Hence, a reference measurement is performed prior to the conveyance of the samples or at a regular suspension of the conveyance of the samples during the measurement. The reference measurement is taken on a reference reflection element, which is set in a light path of an optical measurement system. This provides for improved measurement accuracies of the whole measurement system including the light source and the detector.
In order to regularly set the reference reflection element as reference in the measurement light path, however, a mechanism for automatically conveying the reference reflection element is required in addition to the sample conveyance mechanism. This results in an complicated configuration and an increased cost of the measurement system. Furthermore, the samples must be removed for allowing the reference reflection element to be set to place and then be re-placed at individual places after the reference measurement. This process consumes a part of the time for the sample measurement, resulting in a reduced time for the sample measurement. As a result, the time is used at a lowered efficiency.